This is open source software. Even considering stealth development of a rogue sidechain, the moment they commit the code to a sidechain, the code is out there for everyone to see.
the SC doesn't have to be rogue. it just has to be better.
once it's determined the SC is better, the scBTC become more valuable than BTC. the arb can't work b/c the SC chain is superior and therefore the scBTC are worth more than the BTC. those who moved first will be in a better position. once other BTC holders detect this, they will start to migrate to scBTC. but the catch is: there's a cost to do so. mining fees.
once miners detect this migration they will raise their fees to extortionist levels. they have to b/c accepting a devaluing BTC is risky.
the reason this is more dangerous than simple altcoins is that you're building a system where there is a temptation for miners to merge mine these SC's thus providing them with the security they need to get up and running. we know from Peter Todd that Hill has been aggressively attempting to get mining pools to support SC's.